The present invention generally relates to a semiconductor device for processing a radio frequency (RF) signal, and more particularly relates to an RF device for amplifying power of an RF signal in a mobile communications unit such as a cellular phone.
In recent years, as mobile communications units, e.g., cellular phones, have been popularized, it has become more and more necessary to cut down on the costs of RF devices build in those mobile communications units. Although a known RF device has been implemented using multiple chips (each including a single power-amplifying transistor for an RF signal thereon), a state-of-the-art RF device is implemented using a single chip including multiple power-amplifying transistors thereon.
In this case, those transistors might sometimes affect each another electrically because not so large a space as that of a conventional device is allowed between the transistors. Therefore, there is a problem that the RF characteristic of the overall device degrades compared to the conventional RF device that is implemented using multiple chips.
To solve this problem, a technique of providing an RF signal isolating groove between the adjacent transistors on the substrate of the RF device was developed.
FIGS. 7(a) and 7(b) are respectively a plan view of a known RF device including an RF signal isolating groove and a cross-sectional view of the device taken along the line VIIb-VIIb in FIG. 7(a).
As shown in FIGS. 7(a) and 7(b), first and second power-amplifying FETs (field effect transistors) 202 and 203 are provided on a semiconductor substrate 201. And an RF signal isolating groove 204 is formed between the first and second FETs 202 and 203 in the semiconductor substrate 201. Also, each of the first and second FETs 202 and 203 includes source electrodes 205, a gate electrode 206 and a drain electrode 207.
The RF device shown in FIGS. 7(a) and 7(b) realizes an RF characteristic with rather good linearity, because the RF signal isolating groove 204 is formed between the two FETs 202 and 203 to suppress electrical interference between the FETs 202 and 203.
However, if the RF signal isolating groove 204 shown in FIGS. 7(a) and 7(b) is too shallow, the groove 204 might not function properly in suppressing the electrical interference. On the other hand, if the groove 204 is too deep, the semiconductor substrate 201 might be destroyed.